Let me suggest a slightly different approach.
You are pregnant. You probably want your life to be as stress-free as possible right now.
If that is the most important thing to you, then what you want is the quickest way to bring this matter to an end. From reading the posts on this site, you will see that in general, the quickest way is to accept responsibility, and pay what the railway asks you for - which will be your fare, plus the costs they've incurred. As someone said higher in this thread, that will total somewhere between £80 and £150.
If I have understood things correctly, the railway have your contract details. Make sure you respond to every time they contact you, and make sure that your response is
- short (whoever reads it will be busy and will prefer that you get straight to the point)
- courteous (they are human too - and you want something from them so it's in your interest to be nice to them)
- clear that you know what you did was wrong, and you won't do it again
- willing to cover their costs.
It is probably worth mentioning that you are pregnant, but don't expect that to be a trump card that will guarantee you the railway's sympathy: everyone thinks that their case is special, but for case handlers who spend all day looking at cases, they will see the similarities (that every letter they see is from someone who was caught short faring) more strongly than the differences (pregnant/unwell/bereaved/tired etc., etc.).
Only you can know if you did this by mistake or on purpose: the railway will make a deduction as to why you did it on the evidence they have. You may find it least stressful to do what you can to settle quickly and out of court, even if you don't think that is entirely fair.